1. Field of the Invention
The present invention relates to a charge transfer device and a solid state image sensor using the same, and more particularly to a charge transfer device and a solid state image sensor using the same, capable of transferring signal charge at a high signal to noise ratio (S/N ratio) and preventing an occurrence of dark current.
2. Description of the Prior Art
Recently, charge coupled devices (CCDs) have been used as charge transfer devices having a high S/N ratio. In particular, such CCDs have been applied to solid state image sensors. Such CCDs are classified into surface channel CCDs and buried channel CCDs, in terms of the channel region structure.
Referring to FIGS. 1 and 2, there are illustrated respective sections of a surface channel CCD and a buried channel CCD, and potential profiles according to the sections.
In the drawings, the reference numeral 6 designates a potential profile of each conduction band and the reference numeral 7 designates a potential profile of each valence band. Also, the reference numeral 8 designates a voltage applied to each transfer electrode 5.
In the surface channel CCD shown in FIG. 1, a channel region is formed by inverting the conductivity of the surface of a p type silicon substrate 2a using a voltage 8 applied to the transfer electrode 5 made of polysilicon. In this case, a peak potential is present at a boundary surface between the p type silicon substrate 2a and a first electrode insulating film 4 in the channel region. As a result, an interaction between the potential and the transferred signal charge is actively carried out at the boundary surface, thereby causing the charge transfer efficiency and the S/N ratio to decrease.
On the other hand, in the buried channel CCD shown in FIG. 2, such an interaction between the potential and the transferred signal charge occurs infrequently, since a peak potential is present in the p type silicon substrate 2a.
Accordingly, the buried channel CCD has a lower charge transfer capacity than the surface channel CCD, even though it has a higher S/N ratio than the surface channel CCD.
The primary cause of a dark current generated in the surface channel CCD or the buried channel CCD is the charge generated due to the potential present at the boundary between the p type silicon substrate 2a and the first electrode insulating film
As a result of an improvement in semiconductor material and a cleanliness of fabricating processes and an improvement in photo sensing structure, it has been possible to reduce the dark current to 1 nA/cm.sup.2 even under a condition of a high temperature of 60.degree. C.
However, this value of dark current has been satisfied no longer, since standard signal charge has been recently on decreasing trend, due to increased demands on compactness and high sensitivity of solid state image sensors.
Also, an increase in integration degree results in a decrease in channel region width, so that a high narrow channel effect occurs. Due to such a high narrow channel effect, the quantity of signal charge handled in CCDs is undesirably reduced.
As an effective method for increasing the quantity of handled signal charge, there has been known a method in which a plus voltage and a minus voltage are used as CCD drive voltages. However, this method encounters a problem of an increase in dark current.
In cases of low speed CCDs involving such a problem relating to the dark current, accordingly, only the minus voltage (that is, when electrons are the signal charge) causing no occurrence of dark current has been used.
FIG. 3 illustrates a section, corresponding to one pixel, of a color solid state image sensor using an interline transfer type CCD as its charge transfer device.
In the structure shown in FIG. 3, filter layers 22a to 22c of three colors, namely, red, green and blue are formed above a photodiode 13. The filter layers 22a to 22c serve to output color signals. Also, a photodetector 13 of a certain kind is provided in the structure of FIG. 3. The photodetector 13 may be a pn-junction photodiode.
However, color filters such as red color filter, green color filter and blue color filter are bad in terms of a light utilizing efficiency, since they serve to absorb colors other than desired particular wavelengths which transmitted therethrough.
For realizing a color solid state image sensor with a higher efficiency, accordingly, it is required to provide a technique capable of simultaneously obtaining three color signals with different wavelength bands, namely, a red color signal, a green color signal and a blue color signal. However, it was conventionally impossible to realize a single solid state image sensor capable of simultaneously obtaining color signals with different wavelengths from a received light.
In FIG. 3, the reference numeral 1 designates a n type silicon substrate, 2b a p type well layer substrate, 4 a first electrode insulating film, 10 a channel stop region, 21 and 23 smoothing layers, 25 a p type hole accumulating layer, 26 a photo shield layer, 27 a first polysilicon transfer electrode, 28 a second polysilicon transfer electrode, and 29 a second electrode insulating film.
FIG. 4 is a block diagram of a solid state image sensor including two kinds of photodetectors with different spectral sensitivity formed on a semiconductor substrate. FIG. 5 illustrates a section, corresponding to one pixel, of the solid state image sensor shown in FIG. 4.
In FIG. 5, a first one of the photodetectors is a pn-junction photodiode 13 for a visible range, whereas a second one of the photodetectors is a Schottky barrier diode 12 for an infrared ray range. The Schottky barrier diode 12 is made of PtSi.
In FIG. 5, the reference numeral 2b, 4, 10 and 29 are the same as those of FIG. 3.
The structure shown in FIG. 4 also comprises a plurality of first vertical CCDs 14a for transferring optical signal charge of the visible range and a plurality of second vertical CCDs 14b for transferring optical signal charge of the infrared ray range, so as to read individually signal charge from the photodetectors 13 for the visible range and the photodetectors 12 for the infrared ray range.
Also, the structure comprises a first horizontal CCD 17a for transferring optical signal charge of the visible range from the first vertical CCDs 14a and a second horizontal CCD 17b for transferring optical signal charge of the infrared ray charge from the second vertical CCDs 14b.
In FIG. 4, the reference numeral 29a designates an insulating film for providing an electrical isolation between the first horizontal CCD 17a and the second horizontal CCD 17b.
In such a conventional technique, it is required to provide individual charge transfer paths for the two kinds of photodetectors, that is, the first vertical CCDs, the second vertical CCDs, the first horizontal CCD and the second horizontal CCD. As a result, the conventional structure involves a problem that all signal charge transfer paths should be integrated together in one device.